1. Field of the Invention
This invention relates to a turbine rotor.
2. Description of the Prior Art
In general, a low pressure turbine rotor, or the like, of a large scale steam turbine for driving an electric generator is provided with a plurality of blades which are larger in average diameter than in a high-pressure turbine so as to permit the flow of a large quantity of steam.
In such a huge turbine rotor, to solve problems both in manufacturing techniques and in production economics, there have been developed so-called shrink-fit (or shrunk-on) wheel discs in which the wheel discs to which the blades of respective stages are fitted are previously manufactured separately from the rotor shaft, and thereafter the wheel discs are heated so as to be fitted onto the rotor shaft by shrink fitting.
FIG. 1 illustrates a conventional steam turbine rotor provided with such shrink fit wheel discs. Reference numeral 1 represents a shaft of the turbine rotor. A plurality of shrink-fit wheel discs 3 which are provided with blades 2 on the outer circumference thereof are fitted to the shaft 1 along the axial direction thereof and held with wheel bore keys 4.
There are also installed locking keys 5 which restrain the axial movement of the wheel discs 3.
In the turbine rotor having such a structure, steam energy is converted into torque by means of blades 2, and thereafter is transmitted to the wheel discs 3. The torque transmission from the wheel discs 3 to the shaft 1 is achieved by virtue of friction delivered from the facing pressure of the shrink-fit faces of the wheel discs 3 and the wheel bore keys 4.
In such a turbine rotor, the stage which is nearest the steam inlet side is subjected to relatively higher pressure and temperature. The wheel disc 3 of this stage, with a small heat capacity, is heated faster, by an amount depending upon the operation status thereof, than the shaft 1 so that temperature differences occur between this and adjacent wheel discs 3 and the shaft 1, and this may possibly cause the wheel discs 3 to become loosened. If the wheel discs 3 are loosened, the torque transmission is entirely through the wheel bore keys 4. At the same time, the shrink-fit faces on the inner surfaces of the wheel discs 3 retain some constant facing pressure against the shaft 1 and also maintain a uniform inner face tangential stress.
In the turbine rotor with the above-described structure, on the inner faces of the wheel discs 3 wheel bore key ways 6 are disposed so as to fit the wheel bore keys 4, and stress concentration occurs in the vicinity of the wheel bore key ways 6. As a result, there are dangers of developing stress corrosion cracks (SCC) as indicated by numeral 7 in the vicinity of the wheel bore key ways 6 as shown in FIG. 2. In particular, the wheel discs 3 which are exposed to an atmosphere having temperatures of approximately 130.degree. to 150.degree. C. inevitably develop high stress concentrations, so that the wheel bore key ways 6 are in danger of developing stress corrosion cracking.